1. Field of the Invention
The present invention relates to a push button-operated chucking device for a dental angle piece including clamping elements which press radially against the tool shaft for holding the tool shaft in a frictionally engaging manner. By axially moving an actuating member connected to the push button, and in interaction with a conical control surface, the clamping elements can be moved radially outwardly and inwardly for releasing and clamping the tool shaft.
2. Description of the Related Art
Chucking devices of the above-described type are known in various embodiments.
For example, from DE-C-2 29 05 484 and AT-B 373 488, it is known in the art to support the tool by means of a collet chuck which can be preferably released against the force of a spring by means of a push button arranged at an axial distance from the tool. The chucking action is obtained by the spring in interaction with a conical control surface for the outer side of the collet chuck.
In the device according to DE-C-2 29 05 484, the chucking area is located seen in axial direction near the tool end, i.e., on the side of the push button, while in the device known from AT-B 373 488 the chucking area is near the opening for inserting the tool.
Other means for fixing angle pieces in a frictionally engaging manner are also known in the art. For example, in accordance with Austrian Patent Application A 3448/84, a holding element which is elastic in radial direction by means of a slotted sleeve whose internal radius is smaller than the shaft radius, enlarges its internal radius by the approach of two conical control surfaces, and, thus, releases the tool.
A similar type of support has become known from EP-A1-0 098 754. However, in this case, the change of the radius is achieved only by means of a conical surface.
A different type of fixing device with frictional engagement has become known from EP-A10 420 169. A helical spring mounted in a holder is wound around a clamping member which is slotted in axial direction and presses the clamping member against the tool shaft. When an axial pressure is exerted on the spring, the internal radius of the spring is enlarged and the shaft is released.
All of the above-discussed collet chucks have the significant disadvantage that the axial area in which the shaft is actually clamped, i.e., the clamping area, is relatively short. In addition, in the relatively long axial area in which the collet chucks do not rest against the tool shaft, the tool shaft is guided in a bore of the chucking sleeve, i.e., the guide length, which must have at least a diameter which corresponds to the largest possible diameter of the tool shaft due to tolerances. Since the bore itself is also subject to a tolerance, there is in practically all types of applications a play which may be small, but which is still noticeable because of the high rates of rotation which occur. This play is harmful to the tool and to the handpiece and is unpleasant or even painful to the patient, because it causes wobbling of the tool in its support.
The fixing device according to EP-A1-0 420 169 has the additional disadvantage that during clamping not only the desired decrease of the internal radius of the spring occurs, but also an undesirable increase of the outer diameter thereof, so that, although the tool does not wobble in the clamping sleeve, the spring including tool and clamping sleeve wobbles in the holder. As a result, in spite of the relatively great axial length of clamping between tool and clamping sleeve, fixing with respect to the angle piece again takes place only at one axial location.
Accordingly, all chucking devices have the decisive disadvantage that there is a large difference between the clamping length, i.e., length of contact with the clamping elements, and the guiding length, i.e., insertion length minus clamping length, which facilitates wobbling.
Fixing devices which eliminate this problem have already been proposed in dental hand pieces in which the tool axis coincides with the principal axis of the handpiece and, thus, the axial extension of the tool holder does not play a role.
Thus, AT-B 300 176 discloses a handpiece which includes a double collet chuck which clamps the tool shaft at two axially spaced apart locations. Of course, because of the coaxial arrangement of drive axis and tool axis, the actuation of this collet chuck is only possible by means of complicated rotating mechanisms.
An even more complicated mechanism is disclosed by DD-PS 118 800 because releasing and clamping of the double collet chuck is effected by means of a tilting lever which also rotates in the interior of the handpiece, so that higher rates of rotation are not possible.
Another double collet chuck is known from U.S. Pat. No. 3,631,597. However, in this double collet chuck, only one end is utilized for clamping the tool shaft, while the other end is used for axially fixing the collet chuck.
The use of a true double collet chuck is known from EP-A1-0 421 907. It is a true double collet chuck because it chucks as a result of an axial compression spring and not because of its own elasticity, and because it releases the tool shaft when the spring force is exceeded. As a result, it is possible to clamp the tool shaft at two locations which are axially spaced apart from each other. This double collet chuck has the disadvantage that, although the use of simple conventional conical counterpieces is possible, a lever mechanism for reversing the movement must be provided for the push button because of the construction of the collet chuck as a cylindrical sleeve with two hollow truncated cones which are mounted on the cylindrical sleeve and narrow in axial direction. In addition, the naturally flexible collet chuck has the tendency to buckle under compressive load at high rates of rotation, so that the collet chuck must be guided as exactly as possible along its entire external area.